The use of light emitting diode (LED) based lighting systems has become more commonplace due to their energy savings and significant lifespan. LEDs generate an intense point of light which is generally anisotropic and has a narrow incident beam. The directionality of the light emitted by the LEDs causes excessive glare which can make LEDs very bright and harsh to look at. In some cases, the glare created by LEDs temporarily impairs a person's vision, which makes the use of LEDs for parking lot lamps and street lamps problematic unless proper glare-reducing measures are taken.
An ideal design of an LED lighting system provides sufficient illumination levels on the ground while creating the effect of minimal light at the LED. To help achieve this objective, many LED manufacturers place a primary optic or lens over the semi-conductor element of the LED to create a lambertian light distribution pattern. While this light distribution pattern reduces glare to some degree, some applications, such as roadway lighting, require an even greater amount of glare reduction. In these cases, a secondary optic or lens is placed over each of the LEDs to further distribute the light. Adding the secondary optic, as opposed to modifying the primary optic itself, is preferred because the primary optic is typically installed by the manufacturer and closely integrated with the semi-conductor element of the LED.
The secondary optic typically employs a bubble refraction design that creates a batwing-shaped light distribution pattern in which light rays of greatest intensity extend from a central axis of the secondary optic at a relatively high angle. These high angle light rays, while effective at more evenly illuminating the ground surfaces beneath the luminaire, nevertheless create a significant glare for an individual approaching the luminaire.
To address the high angle brightness of the secondary optic, a tertiary optic or lens is added to diffuse the directional light emitted from the secondary optic. The diffusing characteristic of the tertiary optic disperses light over a larger surface area and thus reduces glare. Known tertiary optics are substantially curved and cover the entire array of the LEDs. As light rays pass through the curved upper ends of the tertiary optic, the light rays are diffracted in the horizontal and upward directions. This results in an undesirable light distribution if the luminaire is to be used outdoors, for example, to illuminate a parking lot or road. It is generally preferred that outdoor luminaries do not emit light in the upward direction because such light tends to exacerbate the problem of light pollution (i.e., the haze of wasted light that envelops many large cities and towns). If the luminaire is configured as a parking lot lamp or street lamp, emitting light in the horizontal direction is also undesirable because doing so may illuminate adjoining properties instead of the intended parking lot surface or road.
Another issue with known curved tertiary optics is that a local minimum or maximum of light intensity is created as the light rays pass through the curvature of the lens. This phenomenon is commonly referred to as pixilation. Pixilation casts shows that can change the look of an illuminated object and potentially create optical illusions.
A need therefore exists for a lighting system incorporating a tertiary optic that reduces glare, and additionally, minimizes light pollution and pixilation.